1. Field of the Invention
The present invention relates generally to integrated circuit device packaging. More particularly, the present invention relates to land grid array (LGA) integrated circuit (IC) device modules.
2. Description of the Related Art
In one known area array IC device packaging and interconnection technique, called land grid array (LGA), electrical connection is established and maintained through pad-to-pad contact by clamping the IC device to a higher assembly such as a printed circuit board through an intervening LGA interposer socket having an array of compliant contacts. LGA technology provides high density, small footprint, low profile IC device interconnections with low inductance for high speed applications. LGA devices avoid thermal mismatch problems, cost less to manufacture and to assemble and can be easily removed and replaced.
In the case of a high density IC device having, for example, 1,089 contacts, the clamping load applied to the top of the IC device may be as high as 300 pounds. Because such IC devices also dissipate a significant amount of heat, a heat sink is required. Attachment of these heat sinks also requires a mechanical load to be applied to the top of the IC device. This mechanical load ensures a low thermal resistance between the IC device and the heat sink and also ensures that the heat sink will remain in place under shock and vibration conditions.
Because of the high clamping loads involved, the relative flexibility of the printed circuit board to which the IC device is electrically connected and the limited ability of the intervening LGA sockets to mate with nonplanar components, a stiffening or backing plate, typically of steel or aluminum, is disposed under the printed circuit board on the side opposite the IC device to provide sufficient stiffness to counteract the mechanical clamping loads applied to the IC device on top of the printed circuit board. In the absence of such a stiffening structure, the printed circuit board can bend or deflect under the applied loads. Excessive bending or deflection of the printed circuit board may cause solder joints and components to fracture or otherwise fail and may also cause LGA sockets, which have limited operating ranges, to lose their electrical connection thereby disrupting electrical communication between the IC device and the associated printed circuit board.
Backing plates are conventionally used with single-sided LGA printed circuit boards, that is, boards carrying LGA IC devices on only one of their major surfaces as exemplified by U.S. Pat. No. 6,198,630 to Cromwell for METHOD AND APPARATUS FOR ELECTRICAL AND MECHANICAL ATTACHMENT, AND ELECTROMAGNETIC INTERFERENCE AND THERMAL MANAGEMENT OF HIGH SPEED, HIGH DENSITY VLSI MODULES, assigned to the assignee of the present invention.
Printed circuit board assemblies in which high density LGA IC devices are mounted on both surfaces of the printed circuit board can present a unique problem in counteracting the substantial clamping loads that must be applied to the IC devices.
FIG. 1 illustrates the problem. FIG. 1 shows a printed circuit board assembly 10 comprising a double-sided printed circuit board 12 having opposed upper and lower major surfaces or sides 14 and 16, respectively. The upper side 14 carries a pair of high density LGA IC devices 18 and 19 and associated LGA interposer sockets 22 and 23, respectively. Similarly, the lower side 16 carries a pair of high density LGA IC devices 20 and 21 and associated LGA interposer sockets 24 and 25, respectively. Because of the circuit routing and interconnection constraints imposed by high density IC devices, the upper IC devices 18 and 19 are offset from the lower IC devices 20 and 21. However, a small amount of overlap, indicated by the reference numeral 28, exists between the upper and lower IC devices. Because of the offset, overlapping arrangement of the. IC devices shown in FIG. 1, it is not possible to provide for each IC device a backing plate on the opposite side of the board that completely backs up the IC device so as to provide adequate stiffness while at the same time serving as a means for receiving clamping fasteners passed through the board from the side carrying the IC device. It will be seen from FIG. 1 that the offset, vertical clamping forces, represented by the arrows 30, applied to the IC devices on the opposite sides of the double-sided printed circuit board 12 can result in local bending or deflection of the printed circuit board as shown by the broken lines 32. Excessive bending or deflection can cause component failure and/or loss of contact between the IC device and the printed circuit board, as already explained.
Accordingly, there is a need to reduce the deflection of such printed circuit board assemblies so as to prevent component failure and maintain reliable electrical contact between the LGA IC devices and the printed circuit board on which they are mounted.
In accordance with one specific, exemplary embodiment of the invention, there is provided an integrated circuit device module comprising a printed circuit board having opposed sides, the printed circuit board comprising a portion carrying an area contact array on one of the sides of the printed circuit board. The module comprises an integrated circuit device having opposed, top and bottom surfaces, the bottom surface of the integrated circuit device comprising an area contact array for electrical communication with the area contact array on the printed circuit board. The module further comprises a clamping mechanism for compressively urging the area contact array on the bottom surface of the integrated circuit device into electrical communication with the area contact array on the portion of the printed circuit board, the forces applied by the clamping mechanism tending to deflect the portion of the printed circuit board and cause the area contact array on the bottom surface of the integrated circuit device to lose electrical communication with the area contact array on the integrated circuit device. A biasing structure is coupled to the printed circuit board for counteracting the forces applied by the clamping mechanism and the resulting tendency of the portion of the printed circuit board to deflect.
In accordance with another specific, exemplary embodiment of the invention, there is provided an integrated circuit device module comprising a printed circuit board having opposed sides; at least one land grid array (LGA) integrated circuit (IC) device mounted on each side of the printed circuit board, the at least one LGA IC device on one side of the printed circuit board being offset from the at least one LGA IC device on the other side of the printed circuit board but overlapping therewith; a clamping mechanism for applying a force to the at least one LGA IC device on each side of the printed circuit board to urge the at least one LGA IC device into electrical communication with an LGA portion of the corresponding side of the printed circuit board under a predetermined load, the predetermined load producing reaction forces which, in combination with the force applied to the LGA IC device, tend to deflect the LGA portion of the printed circuit board; and a biasing structure associated with the at least one LGA IC device on each side of the printed circuit board for applying to the printed circuit board forces counteracting the applied and reaction forces and the resulting tendency for the LGA portion of the printed circuit board to deflect.